Paramecium have been collected from four areas, including two known to be polluted by metals. Bioassays of 48 hour tolerance to cadmium, chromium, cobalt, copper, manganese, mercury, nickel and zinc have revealed within species differences in tolerance to all these metals, except manganese, in at least one of the three species, P. primaurelia, P. biaurelia and P. triaurelia. Generally, but not always, the resistant stocks were collected from areas of known metal pollution. The proposed work will focus on genetic analysis of the within species tolerance differences. The number of loci involved in producing resistance will be determined for each resistant stock. The relationship of the gene(s) producing resistance in different resistant stocks will be determined. The metal specificity of the genes will be studied. The geographical pattern of the tolerance genes will be employed to describe population structure and gene flow in these species.